1. Field of the Invention
The invention relates to the transmission of digital information in which electro-magnetic radiation is modulated in accordance with the information.
2. Description of Related Art
Various detection methods have been proposed in the past for demodulating the transmitted radiation including coherent detection and direct detection. Although coherent detection has major advantages over direct detection, it has the drawback of polarization sensitivity. This problem could, in principle, be eliminated if a transmission medium could be developed which was polarization holding.
In the case of optical radiation, special optical fibres have been developed which are substantially polarization holding but these have complex structures and much higher losses (and are more expensive) when compared to standard circular symmetric monomode fibres. Furthermore, since large quantities of standard optical fibre have already been installed, are being installed, and are planned for the telecommunications which initially use polarization insensitive to direct detection, it is desirable to devise a transmission method and system which is compatible with these fibre coherent networks.
So far two schemes have been proposed that will enable coherent detection, to be used with standard fibre; these are active polarization control and polarization diversity. The former is capable of eliminating all polarization penalties. However, an extra opto-mechanical or electro-optic device is required in either the local oscillator or signal path at the receiver. This complicates the receiver and could also result in an insertion loss penalty. Polarization diversity reception eliminates the need for extra optical control devices in the receiver but requires the addition of a polarising beam splitter and a second photodiode, amplifier chain and intermediate frequency (I.F.) demodulator. With polarization diversity reception there can be up to 3 dB receiver sensitivity penalty for certain combinations of input polarization and local oscillator polarization states when the outputs of the two I.F. demodulators are simply combined (although it may be possible to reduce this penalty to about 1 dB with more complex post demodulator processing).
These two methods of overcoming polarization problems both result in a more complex receiver which although possibly acceptable in a long distance high capacity point-to-point transmission link could introduce a significant cost penalty in a local wideband distribution scheme or LAN/MAN type application.